I think the problem is that you run out of things that are deeply interesting while the "urge to post" continues to grow among users. It seems to me to be an inherent limitation of social news sites that want to keep their front page refreshing with new stuff. As journalists would probably phrase it, it's difficult to fit deep thought into the news cycle.
Some of the "urge to post" comes from the nerdy "somebody's wrong on the Internet" impulse, but a lot of it comes from gamification. HN is just gamified enough to breed competitiveness.
